MFC r347240: Fix dataset name comparison in zfs_compare().

MFC r347240: Fix dataset name comparison in zfs_compare().

The code never returned match comparing two datasets (not snapshots).
As result, uu_avl_find(), called from zfs_callback(), never succeeded,
allowing to add same dataset into the list multiple times, for example:

        # zfs get name pers pers pers@z pers@z
        NAME    PROPERTY  VALUE   SOURCE
        pers    name      pers    -
        pers    name      pers    -
        pers@z  name      pers@z  -

With the patch:

        # zfs get name pers pers pers@z pers@z
        NAME    PROPERTY  VALUE   SOURCE
        pers    name      pers    -
        pers@z  name      pers@z  -